When you talk about summers getting hotter, the first thing people usually think of are hot days. But rising overnight low temperatures are also an important part of the equation. For example, Hartford, Conn. just experienced their warmest July ever without breaking a single daily high temperature record.
As the graphic above makes clear, summer overnight temperatures in Alabama have been rising steadily since 1970, when heat-trapping greenhouse gases began to have a strong effect on Earth’s climate. Click the link below for an interactive chart that looks at these trends from a different angle. This graph shows the number of days each summer when the nighttime temperature stayed above 70 degrees. As you can see, that number has been down over the past 40 years in Mobile (which is not consistent with the trend of warming summer nights nationwide. However, when you look at the overnight temperatures in the state as a whole, you can see those numbers are on the rise.)
Overnight low temperatures have been rising faster than daytime high temperatures thanks to a number of factors. Greenhouse gases, including water vapor, and clouds can play roles in trapping daytime heat and keeping summer nights sultry. With that trend expected to continue across the U.S., rising overnight temperatures will be an important climate change indicator to track.
Rising nighttime temperatures also have a big impact on health. When a hot day is followed by a night where the temperature stays above 80°, the body doesn't have a chance to recover. Without access to air conditioning, the risk of heat exhaustion and heat stroke rises dramatically - especially among the ill and the elderly. The risks are even greater when these days group together into a heat wave, which we are seeing more of in a warming world.
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